The invention relates to a theft protection system, particularly for a shop area, for recording the unauthorized removal of objects or goods to which a preferably passive protection device is affixed having a resonance circuit tuned to a given frequency, through one of a number of exit passages, each of which comprises an antenna system having a transmitting antenna for transmitting electro-magnetic interrogation signals of said given frequency and a receiving antenna for the reasonance signals generated by the unauthorized removal of an object or a good through the passage, an alarm device being connected to a receiver connected with said receiving antenna to generate an alarm in response to reception of said resonance signal.
In recent years theft protection systems of this kind which are used mainly for theft protection of shops have gained increased use and are known in different designs, for instance with the arrangement mentioned above by which the recording of the unauthorized removal of a good from the shop area depends on detection of the electro-magnetic signal when the resonance circuit in a protection device affixed to the good, for instance, in the form of a disc, is influenced by the transmitted electro-magnetic interrogation signals.
In the known systems an antenna system with associated transmitter and receiver is required for each passage to be surveyed, whereby installation of a theft protection system for a shop area having several exit passages will be very expensive.
In addition, the known systems will in many cases not warrant sufficient security against false alarm from an antenna system caused by irrelevant signals such as electrical disturbances or signals from separate antenna systems in other exit passages in the vicinity.
From EPE-A-0003178 a system is known in which a number of exit passages are surveyed by means of separate transmitting/receiving devices controlled according to a master-slave principle and actuated in succeeding time frames.